When commercial 316L stainless steel specimens are heat-treated in a single phase state at 1 100°C, abnormal grain growth (AGG) occurs and some grain boundaries are observed to be faceted with hill-and-valley structures in transmission electron microscopy. Some segments of these faceted grain boundaries are expected to be singular. When heat-treated at 1 300°C normal grain growth occurs with all grain boundaries smoothly curved. These grain boundaries are expected to be atomically rough. At 1 200°C AGG still occurs but there is no excessively large grain as in the specimen heat-treated at 1 100°C. This correlation between the grain boundary structure and grain growth is consistent with those observed previously in pure metals, oxides, and a single phase model alloy. The occurrence of AGG with faceted grain boundaries is attributed to grain boundary movement with boundary steps either produced by two-dimensional nucleation or existing at the junctions with dislocations. As the grain boundaries become rough at 1 300°C normal growth occurs because the grain boundaries migrate continuously with their rate expected to increase linearly with the driving force arising from the size difference. If a specimen heat-treated at 1 100°C is further heat-treated at 1 300°C, the AGG mode appears to switch to normal growth.KEY WORDS: 316L stainless steel; abnormal grain growth; grain boundary faceting.with or without added boron. While apparently ignoring the occurrence of AGG, they observed discontinuous changes in the activation energies for grain growth at about 1 200°C.
Experimental ProcedureA plate of 316L stainless steel of 6 mm in thickness was obtained from Pohang Iron and Steel Company. Its composition is given in Table 1. The plate was cut into approximately 7ϫ7ϫ6 mm pieces, which were heat-treated without any deformation. The specimens were heat-treated at 1 100, 1 200, 1 300°C in a horizontal tube furnace under a flowing Ar atmosphere. They were rapidly pushed into the center of the preheated furnace and quenched in water after the heattreatments. The heat-treated specimens were sectioned through their centers, polished, and etched for microscopic observations. The grain boundaries were examined under a transmission electron microscope (TEM). The grain size was measured by tracing along grain boundaries on micrographs using a digitizer connected to a personal computer. The sizes of about 100-300 grains were measured in each specimen.
Results and DiscussionThe grains in the as-received plate were relatively uniform in size with an average value of about 40 mm, but there were clusters of fine grains in some regions as shown in Fig. 1. The X-ray pole figures did not show any texture in these specimens. The characteristics of grain growth behavior were found to depend on the heat-treatment temperature. During the heat-treatment at 1 100°C, AGG occurred as shown in Fig. 2. After heat-treating for 30 min at 1 100°C some large grains appeared as shown in Fig. 2(b) and these apparently continued to grow to larger sizes durin...
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